The mnemonic command: child mnemonics (closes #4)

keyfunc mnemonic derives a 12, 18 or 24 word child mnemonic through BIP-85's own mnemonic application, with the specification's test vectors as tests. One review round, passed with no findings; the reviewer reproduced the vectors from an implementation written from the specification alone.

Model: opus-5 (implementation and review); fable-5-1 (landing)

internal/cli/cli_test.go

@@ -5,10 +5,12 @@ import (
 	"strings"
 	"testing"
 
+	"git.eeqj.de/sneak/keyfunc/internal/childmnemonic"
 	"git.eeqj.de/sneak/keyfunc/internal/cli"
 	"git.eeqj.de/sneak/keyfunc/internal/derive"
 	"git.eeqj.de/sneak/keyfunc/internal/mnemonic"
 	"github.com/stretchr/testify/require"
+	bip39 "github.com/tyler-smith/go-bip39"
 	"golang.org/x/crypto/ssh"
 )
 
@@ -20,6 +22,12 @@ const (
 		"0I4FKs+eVUulTPHfk9VtXw1tMF"
 )
 
+// The two child mnemonic lengths the tests ask for.
+const (
+	twelve     = 12
+	twentyFour = 24
+)
+
 // example returns the mnemonic the README gives its test vectors for:
 // eleven abandons and about.
 func example() string {
@@ -81,6 +89,28 @@ func TestTheMnemonicCommandIsUsed(t *testing.T) {
 	require.Equal(t, vectorZero+" keyfunc/ssh/0", line)
 }
 
+func TestAChildMnemonicIsPrinted(t *testing.T) {
+	t.Setenv(mnemonic.Variable, example())
+
+	short := strings.Fields(run(t, "mnemonic"))
+	require.Len(t, short, twelve)
+	require.True(t, bip39.IsMnemonicValid(strings.Join(short, " ")))
+
+	long := strings.Fields(run(t, "mnemonic", "--words", "24"))
+	require.Len(t, long, twentyFour)
+
+	next := strings.Fields(run(t, "mnemonic", "-n", "1"))
+	require.NotEqual(t, short, next)
+}
+
+func TestALengthTheToolDoesNotOfferIsRefused(t *testing.T) {
+	t.Setenv(mnemonic.Variable, example())
+
+	out, err := execute(t, "mnemonic", "--words", "15")
+	require.ErrorIs(t, err, childmnemonic.ErrWordCount)
+	require.Empty(t, out)
+}
+
 // run executes the tool with the given arguments and returns what it
 // wrote to standard output.
 func run(t *testing.T, args ...string) string {